Muhammad ibn Yahya from Al-Amraki from Ali ibn Ja'far from his brother (Imam) Musa ibn Ja'far عليه السلام said: I asked Imam عليه السلام about a man praying upon fresh grass that has grown. Imam عليه السلام said: "If he presses his forehead to the ground, then there is no harm." And about praying upon thick grown grass (thayyil) that covers solid ground. Imam عليه السلام said: "There is no harm."